An open bottle of vodka can be kept for decades as it does not go bad. The vodka does get weaker over time. After a few decades, the proof may dip below 25 percent, which increases the possibility of bacteria and yeast developing in the bottle.
Vodka loses its flavor very slowly, so it may taste bad after a decade. Vodka should always be sealed tightly with the original cap. It should be stored in a dark place and not exposed to extreme temperature changes. If the vodka develops an off odor, a strange flavor or a changed appearance, it should be thrown away.